1. Field of the Invention
The present invention generally relates to a method for forming a gradient barrier layer, and more particularly to a method for forming a gradient barrier layer with a composite structure of Ta/TaxN1−x/TaN/TaxN1−x/Ta (tantalum/tantalumx nitride1−x/tantalum nitride/tantalumx nitride1−x/tantalum) for VLSI copper back end of the line (BOEL) technology.
2. Description of the Prior Art
As feature sizes shrink, copper metallization has been proposed to answer the need of high performance and reliable interconnect for high-density integrated circuits since copper has improved stress and electromigration properties and reduced resistivity over the aluminum. However, copper readily diffuses through many materials, including both metals and dielectrics, potentially affecting dielectric constants of insulating material. For example, copper diffusion into the inter-meal dielectric (IMD) such as silicon oxide results in current leakage between adjacent lines and degradation of inter-level dielectric (ILD) breakdown field. Therefore, difficulties with forming copper interconnects have lead to the development of barrier layers that hinder the diffusion of copper into the vulnerable regions.
Referring to FIG. 1, a copper metallization implemented in an integrated circuit technology is illustrated. A barrier layer includes tens nanometers of TaN (tantalum nitride) 108 and Ta (tantalum) 110 sandwiched in between copper layer 112 of the dual damascene structure and an inter-metal dielectric (IMD) 106 such as silicon oxide layer and electrically contacted a copper structure 102 within a substrate 100. In general, the inter-metal dielectric layer 106 is formed on a silicon nitride layer 104 which overlies on the substrate 100 and serves as a passive layer. It is noted that TaN has been proposed as a good copper diffusion barrier and the adhesion of TaN to insulators is adequate, while Ta adheres poorly to oxide-like dielectric but acts better for copper seed formation. Thus, the Ta layer is typically formed on the TaN layer to enhance the adhesion of copper to TaN. In the conventional copper interconnect technology, when oxide-like materials act as the inter-mental dielectric, the adhesion of TaN layer 108 of the barrier layer to copper 102 isn't an issue. Therefore, the conventional barrier layer used in copper back end of the line (BOEL) technology is mainly for preventing copper out-diffusion from the structure 102 and 112 as depicted Arrows.
However, in the new low-k inter-metal dielectric (IMD) material systems, due to the larger thermal expansion coefficients of low-k materials 210 and the poor adhesion of TaN 108 and Cu landing pad 102, the interface 212 of TaN and Cu (108/102) becomes weak and very easy to separate, as shown in FIG. 2. Moreover, the TaN is more brittle and easy to crack. These cause the interconnection open issue and even serious fails in reliability tests such as thermal cycle test (TCT) and stress migration (SM). Therefore, approaches to the adhesion problem induced in the low-k dielectric material systems are prosperously progressing, and the argon (Ar) pre-clean technique is one of many.
The Ar-preclean process has been implemented to removed TaN at via bottom to make Ta film directly contact with Cu surface to increase the adhesion strength. However, due to the TaN layer at the via bottom is extremely thin, the Ar-preclean process margin is very difficult to control. Many side effects, such as micro-trenches 310, materials re-deposition on via sidewall 320, barrier thinning in trench bottom 330, are created and induce more reliability issue as depicted in FIG. 3. Micro-trenches 310 are created due to unevenly over etched. Original via bottom material even including copper residue is re-deposited on via sidewall or diffuses into the low-k dielectric that causes the increase in possibility of electrically discontinuity and changes the characteristic of the low-k dielectric. When the via bottom portion of the TaN layer 108 is removed, a partial of TaN layer 108 at trench bottom is also removed resulting in barrier thinning problem or, even worse, no TaN layer reserved, as respectively indicated by reference numbers 330 and 340.
In view of the prior art described, it is a desire to provide a barrier layer with a low diffusion coefficient for metal conductive layers, excellent adhesion and more tensile properties, and more uniform step coverage characteristic.